Trimethylamine N-oxide (TMAO) is an organic osmolyte present at high levels in elasmobranchs, in which it counteracts the deleterious effects of urea on proteins, and is also accumulated by deep-living invertebrates and teleost fishes. To test the hypothesis that TMAO may compensate for the adverse effects of elevated pressure on protein structure in deep-sea species, we studied the efficacy of TMAO in preventing denaturation and enhanced proteolysis by hydrostatic pressure. TMAO was compared to a common ‘compatible’ osmolyte, glycine, using muscle-type lactate dehydrogenase (A(4)-LDH) homologs from three scorpaenid teleost fish species and from a mammal, the cow. Test conditions lasted 1 h and were: (1) no addition, (2) 250 mmol l(−)(1) TMAO and (3) 250 mmol l(−)(1) glycine, in the absence and presence of trypsin. Comparisons were made at 0. 1 and 101.3 MPa for the deeper occurring Sebastolobus altivelis, 0.1, 50.7 and 101.3 MPa for the moderate-depth congener S. alascanus, 0. 1 and 25.3 MPa for shallow-living Sebastes melanops and 0.1 and 50.7 MPa for Bos taurus. Susceptibility to denaturation was determined by the residual LDH activity. For all the species and pressures tested, 250 mmol l(−)(1) TMAO reduced trypsinolysis significantly. For all except S. altivelis, which was minimally affected by 101.3 MPa pressure, TMAO stabilized the LDH homologs and reduced pressure denaturation significantly. Glycine, in contrast, showed no ability to reduce pressure denaturation alone, and little or no ability to reduce the rate of proteolysis.

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